We added our company logo to outgoing emails and then noticed that our emails weren't going out from nopCommerce and we received something like the following in the System Log:
Error sending e-mail. Mailbox unavailable. The server response was: Your message was marked as spam and scored 5.3 points with a required score

Our hosting vendor pointed us to the following article:
http://www.andreas-kraus.net/blog/tips-for-avoiding-spam-filters-with-systemnetmail/

Based on that, I changed the Nop.BusinessLogic.MessageManager class' SendEmail function as follows:
1.  Comment out "message.Body = Body;"
2.  Comment out "message.IsBodyHtml = true;"
3.  Below that line, insert the following:

            message.BodyEncoding = System.Text.Encoding.GetEncoding("utf-8");

            if (!Body.StartsWith("<html>", true, CultureInfo.CurrentCulture))
            {
                Body = "<html><body>" + Body + "</body></html>";
            }

            AlternateView plainView = AlternateView.CreateAlternateViewFromString(
                System.Text.RegularExpressions.Regex.Replace(Body, @"<(.|\n)*?>", string.Empty), null, "text/plain");
            AlternateView htmlView = AlternateView.CreateAlternateViewFromString(Body, null, "text/html");

            message.AlternateViews.Add(plainView);
            message.AlternateViews.Add(htmlView);


After doing this, all our our emails go out.

We help this helps someone else out.

Thanks,
Ben